Frequency division multiplexing (FDM) is a technology for transmitting different data sets within each of multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within a carrier—a unique frequency range that is modulated by data being transmitted.
Orthogonal frequency division multiplexing (OFDM) is a spread spectrum technique that distributes each data set of the different data sets over a large number of carriers that are spaced apart at predetermined frequencies. This spacing provides the “orthogonality” in this technique, which allows for demodulators that are responsive only to frequencies relating to a signal data set. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. OFDM is advantageous because in a typical terrestrial broadcasting scenario there are multipath-channels—transmitted signals arrive at a receiver using various paths of different length. Since multiple versions of a signal interfere one with another it becomes difficult to extract data being transmitted.
For amplifying an OFDM signal, an amplifier must support a range of pulse amplitudes from a first level of amplitude through a peak amplitude. Though support for peak amplitude is a requirement of OFDM standards, OFDM peak pulses come with such infrequency that designing an amplifier to support them, though required, increases the power consumption of the PA and adds a level of complexity and cost that is undesirable.
For example, it is known to improve power consumption of PAs by varying supply voltage with a DC-to-DC converter to be proportional to the amplitude of the transmitted signal as depicted in WO040086321. Lower collector voltages are used to achieve lower output powers and higher collector voltages are used to deal with higher amplitude portions of the signal to be transmitted. Assuming high efficiencies in the regulator, very low power consumption is realized at low modulation frequencies by switchmode techniques. As the frequency of the modulation is increased the difficulty of designing ever faster switchmode regulators becomes too great and linear amplification stages are needed and no efficiency gain results.
The regulator approach is Prior Art, an example of which is depicted in U.S. Pat. No. 685320, and is an effective way of increasing the PA efficiency, by varying the collector or drain voltage on the amplifying transistor and changing the load line of the PA. Linearity requirements, however, force the gain/phase response to be linear with voltage change, or that pre-distortion is applied.
Other variants on this theme attempt to use a very fast, envelope tracking power supply on the collector in combination with a variable base supply. The modulation amplitude is realized by varying the power supply voltage while the phase information is injected onto the RF signal. Envelope tracking requires an even more complex power supply than the DC-to-DC converter approach and has yet to be demonstrated in a practical fashion. Such a circuit is disclosed in U.S. Pat. No. 6,437,641 where an excess envelope sensor is used to detect peak voltages and in turn enhance the output voltage fed to the power amplifier.
Several RF systems containing power amplifiers have recurrent periods with large peak excursions and these peak excursions need to be handled, in order to improve the efficiency of these systems by ensuring linearity of the power amplifier. As discussed in WO 01/67598, in prior art applications, one method to handle signals with large peak-to-average ratios is to control the DC power supply to a power amplifier. For example, in this case, one set of voltage levels are supplied to the power amplifier when the instantaneous amplitude is below a desired level and another set of voltage levels is supplied when the instantaneous amplitude is above a desired level.
In prior art, U.S. Pat. No. 6,831,519 a circuit is disclosed for allowing a power amplifier to work under different operating power supply voltages, in response to different input signals. Specifically, circuitry is provided for controlling the level of the voltage supplied to a power amplifier, via supplementary supply path. In this circuit, two controllable impedances in the form of field-effect transistors as well as an inductor are provided, for when in operation with control pulses, directing the flow of current and providing enhancement voltage to the amplifier when necessary. However, the disadvantage of such a circuit is that it is not realizable within a low cost integrated power amplifier due to the difficulty in integrating the inductor and the PA on the same substrate.
It would be advantageous to provide a method and apparatus to improve the power output, efficiency, and distortion of an OFDM power amplifier without significantly increasing the power supply complexity, or needing a second voltage supply. Advantageously, improving these attributes is beneficial in WLAN systems in order to provide users with better data transmission range, longer intervals between battery charging, and more generally lower power consumption.